Differently from visible light, normal incidence optics is hardly usable for X-rays. For this reason, taking advantage of the fact that a refractive index of metal with respect to an X-ray is less than one, a grazing-incidence optics based on total reflection on a metal surface is used for X-rays. In this case, a critical angle for the total reflection is as small as about 1 degree. Thus, as means to obtain a larger effective area of a reflecting surface, there has been known a technique of concentrically arranging a large number of cylindrical-shaped metal reflecting mirrors different in diameter. However, this technique causes an increase in overall weight of an X-ray reflecting device, so that the X-ray reflecting device will be of difficult to transport from the earth for use in cosmic space.
Moreover, in order to ensure reflectance at a certain level or more, the smoothness of a surface of each reflecting mirror in the X-ray reflecting device is required to be comparable to the wavelength of an X-ray. Therefore, in the X-ray reflecting device, there has been a need for subjecting the reflecting surface to polishing so as to smooth the surface. Thus, for example, after preparing a large number of replica mirrors by pressing a thin film onto a polished master die, reflecting mirrors have been produced one by one while spending a lot of time and effort (see the following Non-Patent Document 1). As means for reducing the weight of the mirror, there has also been known a technique of using a thin aluminum foil as a mirror. However, this technique has an disadvantage of causing deterioration in focusing performance due to deformation or distortion of the foil (see the Non-Patent Document 1).
Therefore, a group of the European Space Research and Technology Centre (ESTEC) of the European Space Agency (ESA) has proposed a technique of using a surface-polished silicon wafer as an X-ray reflecting mirror (see the following Non-Patent Document 2). A surface of a commercially-available polished silicon wafer has angstrom-level smoothness, and thereby can be directly used as an X-ray reflecting mirror. A wafer surface is capable of being finished to an extremely precise flatness, and therefore is excellent in focusing performance. A silicon wafer has a thickness approximately equal to that of an aluminum foil, and therefore can provide a relatively lightweight optics.
In cases where an optics is made by the technique described in the Non-Patent Document 2, a silicon wafer is subjected to press-bending, i.e., elastic deformation, to have a shape close to an ideal curved surface, and then a large number of mirrors are formed side-by-side in a concentric arrangement. However, in the silicon wafer subjected to elastic deformation, due to slight shifting of a pressing direction caused by fine dust trapped between a pressing member and the silicon wafer, aging, temperature change, etc., a deviation occurs in a curved surface shape of the mirror, which causes a problem of instability in focusing performance.
[Non-Patent Document 1] T. Namioka, K. Yamashita, “X-ray Crystal Optics”, BAIFUKAN Co., Ltd. (pp. 136-143, etc) (concerning conventional X-ray reflecting devices and multilayer reflecting mirrors)
[Non-Patent Document 2] Bavdaz et al., 2004, Proc. of SPIE, 5488, 829 (concerning an X-ray optics using a surface-polished silicon wafer in an elastically deformed state)
[Non-Patent Document 3] Nakajima et al., 2005, Nature Materials, 4, 47 (concerning an optics utilizing Bragg reflection and thermal plastic deformation of a silicon wafer) [Non-Patent Document 4] Sato & Tonehara, 1994, applied Physics Letter, 65, 1924 (concerning surface smoothing of a silicon wafer by hydrogen annealing)